using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        /*

        Deferred execution means that the evaluation of an expression is delayed until its realized value is actually required. 

        It greatly improves performance by avoiding unnecessary execution.

        You can implement deferred execution for your custom extension methods for IEnumerable using the yield keyword of C#.

        */

        IList<Student> studentList = new List<Student>() 

        { 

            new Student() { StudentID = 1, StudentName = "John", Age = 13 } ,

            new Student() { StudentID = 2, StudentName = "Steve",  Age = 15 } ,

            new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,

            new Student() { StudentID = 4, StudentName = "Ram" , Age = 12 } ,

            new Student() { StudentID = 5, StudentName = "Ron" , Age = 21 } 

        };

        var teenAgerStudents =  studentList.GetTeenAgerStudents();

​

        foreach(var teenStudent in teenAgerStudents)

            Console.WriteLine("Student Name: {0}", teenStudent.StudentName);

    }

}

​

public class Student 

{

    public int StudentID { get; set; }

    public string StudentName { get; set; }

    public int Age { get; set; }

}

​

public static class EnumerableExtensionMethods

{

    public static IEnumerable<Student> GetTeenAgerStudents(this IEnumerable<Student> source)

    {

        foreach (Student std in source)

        {

            Console.WriteLine("Accessing student {0}", std.StudentName);

            if (std.Age > 12 && std.Age < 20)

                yield return std;

        }

    }

}